FIG. 11 is a circuit diagram showing a simplified conventional large-area imaging system (“imager”) 50, which is a well-known device that utilizes a large-area sensor array and associated electronics to capture radiation images (e.g., visible light images, or high-energy X-ray images). Conventional large imaging system 50 includes a large-area sensor substrate 60 (e.g., glass or quartz) having formed thereon pixels 61 that are arranged in rows and columns, and a separate control/readout circuit 70 that is used to drive and read image data from sensor substrate 60 during imager operation. Each pixel 61 typically includes a sensor (radiation detection element) 62 and an access transistor 63, which is typically a thin film transistor (TFT) formed on substrate 60. Each sensor typically includes a photosensitive region (e.g., a p-i-n photodiode formed using amorphous silicon (a-Si)) that is formed over access transistors 62, and is connected to a bias voltage Vbias. As indicated in FIG. 11, the access transistors in each column of pixels are accessed by an associated word line 64, and data read from the sensors in each row of pixels is passed to an associated data line 65 during readout operations. When exposed to a radiation pattern (e.g., an x-ray image), each sensor 62 generates a sensor charge that is representative of the quantity of ionizing radiation incident on that sensor. When accessed during a sensor readout operation, the sensor signal is transmitted on an associated data line 64 to control/readout circuit 70, which amplifies and/or converts the sensor signal to form digital image data. This digital image data is then stored with image data read from all pixels of the array, and can be used to produce an image representing the original radiation image.
Several problems are associated with conventional large-area imaging systems.
First, conventional large-area imaging systems typically require that sensor substrate 60 and control/readout substrate 70 be fabricated separately, and connected by a large number of connections. The large-area process used to manufacture sensor substrate 60 cannot produce the conventional integrated circuitry associated with control/readout circuit 70, thereby requiring separate sensor substrate 60 and control/readout circuit 70 to be formed on separate substrates using different fabrication techniques. After being produced by separate fabrication techniques, sensor substrate 60 and control/readout circuit 70 are then combined during production using wires or other connectors 75. The large number of required connectors 75 gives rise to increased production costs and other associated problems, such as limited image quality due to data-line noise pickup and/or kTC noise, electromechanical reliability, and production yield. Further, the number of required connections 75 is associated with the size of the sensor array (i.e., one connection per word line, one connection per data line, plus additional connections for bias voltage Vbias, etc.). Therefore, increasing the number of pixel rows and/or columns to increase the imager size or resolution involves increasing the number of connections, which in turn increases production costs and the other associated problems mentioned above.
Another problem associated with conventional large-area imaging systems is that data line capacitance and switching noise, which increase with the pixel array size, are the dominant noise sources in the imager system. Conventional imagers drain sensor charges onto associated data lines during readout. The longer data lines associated with large-area sensor arrays represent a greater data-line capacitance, which in turn requires larger access transistors to facilitate readout of the data in a timely manner. The large access-transistor gates cause a high gate-line feed-through charge and associated noise. The larger data-line capacitance increases the associated kTC noise. Both noise sources greatly reduce the accuracy of the read out data and dominate the overall imager system performance. Reducing the size of the access transistors may reduce this noise term, but also decreases readout speed.
Accordingly, what is needed is a large-area imaging system in which the number of connections between a sensor substrate and control substrate is minimized, and in which the dominant noise effects and the readout speed are independent of the array size.